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Cardiomyocyte sarcomeres contain localized ribosomes, but the factors responsible for their localization and the significance of localized translation are unknown. Using proximity labeling, we identified Ribosomal Protein SA (RPSA) as a Z-line protein. In cultured cardiomyocytes, the loss of RPSA led to impaired local protein translation and reduced sarcomere integrity. By employing CAS9 expressing mice along with adeno-associated viruses expressing CRE recombinase and single-guide RNAs targeting Rpsa, we knocked out Rpsa in vivo and observed mis-localization of ribosomes and diminished local translation. These genetic mosaic mice with Rpsa knockout in a subset of cardiomyocytes developed dilated cardiomyopathy, featuring atrophy of RPSA-deficient cardiomyocytes, compensatory hypertrophy of unaffected cardiomyocytes, left ventricular dilation, and impaired contractile function. We demonstrate that RPSA C-terminal domain is sufficient for localization to the Z-lines and that if the microtubule network is disrupted RPSA loses its sarcomeric localization. These findings highlight RPSA as a ribosomal factor essential for ribosome localization to the Z-line, facilitating local translation and sarcomere maintenance.

Translational control of gene expression is crucial in cardiomyocytes, particularly in response to hypertrophic stimuli. The extracellular signal-regulated kinases (ERK) pathway plays a key role in inducing cardiac hypertrophy and in regulating specific protein translation. However, it is unclear how the specificity is achieved, and the spatiotemporal dynamics of protein translation remain unexplored. We employed single-molecule imaging of nascent peptides (SINAPs) reporters to visualize and analyze the translation dynamics in single adult rat ventricular cardiomyocytes and tracked active translation sites at high spatiotemporal resolution. We also examined the effects of adrenergic stimulation and the role of the ERK pathway in translation localization. Our findings revealed that translation sites are primarily localized near Z-lines in cardiomyocytes, with some sites being highly dynamic and moving during translation. The 3’ untranslated regions did not significantly change the localization of translation. Many translation sites co-localized with microtubules, and their movement predominantly occurred along microtubular tracks. Adrenergic stimulation led to a transient shift in translation activity toward the peri-nuclear region, peaking at 12 hours and requiring ERK pathway activity for this localization change. Our high-resolution single-cell study demonstrates that protein translation in cardiomyocytes is dynamic and responsive to hypertrophic stimuli in an ERK-dependent manner. The localized translation mechanism allows cardiomyocytes to rapidly adapt to changing environments by preferentially translating mRNAs in the peri-nuclear region. These findings provide new insights into the spatial regulation of translation in cardiomyocytes and its role in cardiac hypertrophy.

Cardiomyocyte sarcomeres contain localized ribosomes, but the factors responsible for their localization and the significance of localized translation are unknown. Using proximity labeling, we identified Ribosomal Protein SA (RPSA) as a Z-line protein. In cultured cardiomyocytes, the loss of RPSA led to impaired local protein translation and reduced sarcomere integrity. By employing CAS9 expressing mice along with adeno-associated viruses expressing CRE recombinase and single-guide RNAs targeting Rpsa, we knocked out Rpsa in vivo and observed mis-localization of ribosomes and diminished local translation. These genetic mosaic mice with Rpsa knockout in a subset of cardiomyocytes developed dilated cardiomyopathy, featuring atrophy of RPSA-deficient cardiomyocytes, compensatory hypertrophy of unaffected cardiomyocytes, left ventricular dilation, and impaired contractile function. We demonstrate that RPSA C-terminal domain is sufficient for localization to the Z-lines and that if the microtubule network is disrupted RPSA loses its sarcomeric localization. These findings highlight RPSA as a ribosomal factor essential for ribosome localization to the Z-line, facilitating local translation and sarcomere maintenance.

How the sarcomeric complex is continuously turned-over in long-living cardiomyocytes is unclear. According to the prevailing model of sarcomere maintenance, sarcomeres are maintained by cytoplasmic soluble protein pools with free recycling between pools and sarcomeres. We imaged and quantified the turnover of expressed and endogenous sarcomeric proteins, including the giant protein titin, in cardiomyocytes in culture and in vivo, at the single cell and at the single sarcomere level using pulse-chase labeling of Halo-tagged proteins with covalent ligands. We disprove the prevailing ‘protein pool’ model and instead show an ordered mechanism in which only newly translated proteins enter the sarcomeric complex while older ones are removed and degraded. We also show that degradation is independent of protein age, and that proteolytic extraction is a rate limiting step in the turnover. We show that replacement of sarcomeric proteins occurs at a similar rate within cells and across the heart and is slower in adult cells. Our findings establish a ‘unidirectional replacement’ model for cardiac sarcomeres subunit replacement and identify their turnover principles.